DHA is a polyunsaturated fatty acid contained abundantly in phospholipids in the brain and the like of mammals including human beings and is known to play an important role in the maintenance or development of brain functions. Human beings have an ability to synthesize DHA from linolenic acid, however, the amount thereof is much smaller than the required amount, and it is known that DHA should be taken from an outside. For this reason, many foods, supplements, and milk for babies containing DHA are sold.
Examples of DHA contained in these products include DHA obtained by extraction from fish oils. Fish oils are inexpensive and rich in DHA, and therefore are good sources for DHA. However, in consideration of a recent increase in demand for DHA, accumulation of PCB or a heavy metal in fish accompanying marine pollution, etc., DHA to be supplied more safely and stably has been demanded.
Other than DHA obtained by extraction from fish oils, the examples include DHA produced by a fermentation method using a microorganism. It has been known for a long time that marine microalgae produce a lipid containing DHA (Non-Patent Document 1). However, there are problems that the amount of DHA contained in microalgae is small, high-density cultivation is difficult, etc., and therefore, production at a commercial level was not performed.
As a method for producing DHA by fermentation at a commercial level, the case using dinoflagellate Crypthecodinium cohnii is the first study to report the method (Patent Document 1 and Non-Patent Documents 2 and 3). Thereafter, multiple microorganisms having an ability to produce DHA were discovered, and microorganisms exhibiting the highest ability to produce DHA are Labyrinthulea microorganisms, and examples of such microorganisms include Schizochytrium sp. ATCC 20888 strain and derivative strains thereof (Patent Documents 2 and 3), Aurantiochytrium limacinum (formerly classified as Schizochytrium limacinum) SR21 strain (Patent Documents 4 to 6 and Non-Patent Document 4), Labyrinthulea microorganism 12B strain (Patent Document 7), Thraustochytrium sp. LEF strain (Patent Document 8), and the like.
Among these Labyrinthulea microorganisms, a microorganism exhibiting the highest ability to produce DHA is a Schizochytrium sp. S31 strain (ATCC 20888 strain) (Patent Document 9). The ability to produce DHA of the microorganism is overwhelmingly higher as compared with other reports, and a fermentation method for DHA using the microorganism can be said to be a fermentation production process with the lowest cost.
However, even with the fermentation production process, the cost is higher than with the extraction method from fish oils, and therefore, further improvement of productivity has been demanded.